Methods and Circuitry for Mitigating Saturation in Wireless Power Systems

ABSTRACT

A wireless charging system having a power transmitter may wirelessly transfer power to a power receiver. Shield saturation, such as saturation of a ferrite structure, in the wireless power receiver may occur under some operating conditions. Saturation can lead to disruptive oscillations in power transfer. The power transmitting may include control circuitry for detecting and mitigating saturation.

This application claims the benefit of provisional patent applicationNo. 63/143,704, filed Jan. 29, 2021, which is hereby incorporated byreference herein in its entirety.

FIELD

This relates generally to power systems, and, more particularly, towireless power systems for charging electronic devices.

BACKGROUND

In a wireless charging system, a wireless power transmitting device suchas a charging mat wirelessly transmits power to a wireless powerreceiving device such as a battery-powered, portable electronic device.The wireless power transmitting device has a coil that produceselectromagnetic flux. The wireless power receiving device has a coil andrectifier circuitry that uses electromagnetic flux produced by thetransmitter to generate direct-current power that can be used to powerelectrical loads in the battery-powered portable electronic device. Itcan be challenging to design a wireless charging system.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a block diagram of an illustrative wireless charging systemthat includes a wireless power transmitting device and a wireless powerreceiving device in accordance with some embodiments.

FIG. 1B is an exploded view of an illustrative wireless power receivingdevice in accordance with some embodiments.

FIG. 2 is a circuit schematic of wireless power transmitting andreceiving circuitry in accordance with some embodiments.

FIG. 3 is a plot showing a reduction in mated inductance caused bysaturation in accordance with some embodiments.

FIG. 4 is a timing diagram illustrating the behavior of a transmit coilcurrent in a wireless power transmitting device with and withoutsaturation in accordance with some embodiments.

FIG. 5 is a flow chart of illustrative steps for performing power rampup in accordance with some embodiments.

FIG. 6 is a flow chart of illustrative steps for performing saturationdetection and mitigation in accordance with some embodiments.

FIG. 7 is a circuit diagram of an inverter driving a resonant circuit inaccordance with some embodiments.

FIG. 8A is a timing diagram showing an inverter output with a 180° phaseshift and a symmetric switching duty cycle in accordance with someembodiments.

FIG. 8B is a timing diagram showing an inverter output with a 180° phaseshift and an asymmetric duty cycle in accordance with some embodiments.

FIG. 8C is a timing diagram showing an inverter output with a 90° phaseshift and an asymmetric duty cycle in accordance with some embodiments.

DETAILED DESCRIPTION

A wireless power system includes a wireless power transmitting device.The wireless power transmitting device wirelessly transmits power to oneor more wireless power receiving devices. The wireless power receivingdevices may include electronic devices such as wristwatches, cellulartelephones, tablet computers, laptop computers, ear buds, battery casesfor ear buds and other devices, tablet computer styluses (pencils) andother input-output devices, wearable devices, or other electronicequipment. The wireless power transmitting device may be an electronicdevice such as a wireless charging mat or puck, a tablet computer orother battery-powered electronic device with wireless power transmittingcircuitry, or other wireless power transmitting device. The wirelesspower receiving devices use power from the wireless power transmittingdevice for powering internal components and for charging an internalbattery. Because transmitted wireless power is often used for charginginternal batteries, wireless power transmission operations are sometimesreferred to as wireless charging operations.

An illustrative wireless power system, sometimes referred to as awireless charging system, is shown in FIG. 1A. As shown in FIG. 1A,wireless power system 8 includes a wireless power transmitting devicesuch as wireless power transmitting device 12 and includes a wirelesspower receiving device such as wireless power receiving device 24.Wireless power transmitting device 12 includes control circuitry 16.Wireless power receiving device 24 includes control circuitry 30.Control circuitries in system 8 such as control circuitry 16 and controlcircuitry 30 are used in controlling the operation of system 8. Thiscontrol circuitry may include processing circuitry associated withmicroprocessors, power management units, baseband processors,application processors, digital signal processors, microcontrollers,battery chargers, and/or application-specific integrated circuits withprocessing circuits. The processing circuitry implements desired controland communications features in devices 12 and 24.

For example, the processing circuitry may be used in selecting wirelesspower coils, determining power transmission levels, processing sensordata and other data, processing user input, handling negotiationsbetween devices 12 and 24, sending and receiving in-band and out-of-banddata, making measurements, and otherwise controlling the operation ofsystem 8. As another example, the processing circuitry may include oneor more processors such as an application processor that is used to runsoftware such as internet browsing applications,voice-over-internet-protocol (VOIP) telephone call applications, emailapplications, media playback applications, operating system functions,power management functions for controlling when one or more processorswake up, game applications, maps, instant messaging applications,payment applications, calendar applications, notification/reminderapplications, and so forth.

Control circuitry in system 8 may be configured to perform operations insystem 8 using hardware (e.g., dedicated hardware or circuitry),firmware and/or software. Software code for performing operations insystem 8 is stored on non-transitory computer readable storage media(e.g., tangible computer readable storage media) in control circuitry 8.The software code may sometimes be referred to as software, data,program instructions, instructions, or code. The non-transitory computerreadable storage media may include non-volatile memory such asnon-volatile random-access memory (NVRAM), one or more hard drives(e.g., magnetic drives or solid state drives), one or more removableflash drives or other removable media, or the like. Software stored onthe non-transitory computer readable storage media may be executed onthe processing circuitry of control circuitry 16 and/or 30. Theprocessing circuitry may include application-specific integratedcircuits with processing circuitry, one or more microprocessors such asan application processor, a central processing unit (CPU) or otherprocessing circuitry.

Wireless power transmitting device 12 may be a stand-alone power adapter(e.g., a wireless charging mat or puck that includes power adaptercircuitry), may be a wireless charging mat or puck that is coupled to apower adapter or other equipment by a cable, may be a battery-poweredelectronic device (cellular telephone, tablet computer, laptop computer,removable case, etc.), may be equipment that has been incorporated intofurniture, a vehicle, or other system, or may be other wireless powertransfer equipment. Illustrative configurations in which wireless powertransmitting device 12 is a wireless charging puck or battery-poweredelectronic device are sometimes described herein as an example.

Wireless power receiving device 24 may be a portable electronic devicesuch as a wristwatch, a cellular telephone, a laptop computer, a tabletcomputer, an accessory such as an earbud, a tablet computer input devicesuch as a wireless tablet computer stylus (pencil), a battery case, orother electronic equipment. Wireless power transmitting device 12 mayinclude one or more input-output devices 62 (e.g., input devices and/oroutput devices of the type described in connection with input-outputdevices 56) or input-output devices 62 may be omitted (e.g., to reducedevice complexity). Wireless power transmitting device 12 may be coupledto a wall outlet (e.g., an alternating current power source), may have abattery for supplying power, and/or may have another source of power.Device 12 may have an alternating-current (AC) to direct-current (DC)power converter such as AC-DC power converter 14 for converting AC powerfrom a wall outlet or other power source into DC power.

In some configurations, AC-DC power converter 14 may be provided in anenclosure (e.g., a power brick enclosure) that is separate from theenclosure of device 12 (e.g., a wireless charging puck enclosure orbattery-powered electronic device enclosure) and a cable may be used tocouple DC power from the power converter to device 12. DC power may beused to power control circuitry 16. During operation, a controller incontrol circuitry 16 may use power transmitting circuitry 52 to transmitwireless power to power receiving circuitry 54 of device 24. Powertransmitting circuitry 52 may have switching circuitry (e.g., invertercircuitry 60 formed from transistors) that is turned on and off based oncontrol signals provided by control circuitry 16 to create AC currentsignals through one or more transmit coils 42. Coils 42 may be arrangedin a planar coil array (e.g., in configurations in which device 12 is awireless charging mat) or may be arranged to form a cluster of coils(e.g., in configurations in which device 12 is a wireless chargingpuck). In some arrangements, device 12 (e.g., a charging mat, puck,battery-powered device, etc.) may have only a single coil. In otherarrangements, wireless charging device 12 may have multiple coils (e.g.,two or more coils, 5-10 coils, at least 10 coils, 10-30 coils, fewerthan 35 coils, fewer than 25 coils, or other suitable number of coils).

As the AC currents pass through one or more coils 42, the coils 42produce electromagnetic field signals 44 in response to the AC currentsignals. Electromagnetic field signals (sometimes referred to aswireless power signals) 44 can then induce a corresponding AC current toflow in one or more nearby receiver coils such as coil 48 in powerreceiving device 24. When the alternating-current electromagnetic fieldsare received by coil 48, corresponding alternating-current currents areinduced in coil 48. Rectifier circuitry such as rectifier 50, whichcontains rectifying components such as synchronous rectificationmetal-oxide-semiconductor transistors arranged in a bridge network,converts received AC signals (received alternating-current signalsassociated with electromagnetic field 44) from coil 48 into DC voltagesignals for powering loads in device 24 such powering applicationprocessors as well as charging a battery in the device. This principleof wireless power transfer can be referred to as the transmitting andreceiving of wireless power signals.

The DC voltages produced by rectifier 50 can be used in powering anenergy storage device such as battery 58 and can be used in poweringother components in device 24. For example, device 24 may includeinput-output devices 56 such as a display, touch sensor, communicationscircuits, audio components, sensors, components that produceelectromagnetic signals that are sensed by a touch sensor in tabletcomputer or other device with a touch sensor (e.g., to provide stylusinput), and other components and these components may be powered by theDC voltages produced by rectifier 50, in combination with otheravailable energy sources such as battery 58.

During wireless power transmission operations, circuitry 52 supplies ACdrive signals such as AC current signals to one or more coils 42 at agiven power transmission frequency. The power transmission frequency issometimes referred to as a carrier frequency, power carrier frequency,drive frequency, or inverter switching frequency Fs. The inverterswitching frequency Fs may be, for example, a predetermined frequency ofabout 125 kHz, about 128 kHz, about 200 kHz, about 326 kHz, about 360kHz, at least 80 kHz, at least 100 kHz, less than 500 kHz, less than 300kHz, or other suitable wireless power frequency. Devices operating underthe Qi wireless charging standard established by the Wireless PowerConsortium generally operate between 110-205 kHz or between 80-300 kHz.In some configurations, the switching frequency Fs is negotiated incommunications between devices 12 and 24. In other configurations, thepower transmission frequency can be fixed.

Control circuitry 16 may also include external object measurementcircuitry 41 configured to detect external objects on a charging surfaceof device 12 and to make other desired measurements such as currentmeasurements, voltage measurements, power measurements, and/or energymeasurements. Measurement circuitry 41 can detect indications of objectsabutting device 12. Measurement circuitry 41 can aid in the detection ofwhether a nearby object is compatible with wireless charging operations,or if the nearby object is likely a foreign object such as coils, paperclips, coins, and other generally metallic objects that react toinductive fields but incompatible with wireless charging.

FIG. 1B shows an exploded view of power receiving device 24. As shown inFIG. 1B, exemplary power receiving device 24 includes a device housingsuch as housing layer 300, wireless power coil 48, shielding layers 302and 304, battery 58, display 306, and a cover layer such as cover glass308 disposed over display 306. Device housing 300 and cover glass 308serve as lower and upper external protective layers, respectively.Although not explicitly shown, additional components such ascommunications, storage, and processing components are included withinthe stack-up of device 24. The arrangements of components in a devicesuch as device 24 may vary.

Electronic components within device 24 are subject to signalinterference. Shielding layer 302 can be a metal shield configured tosuppress electromagnetic interference. Shielding layer 302 of this typecan be formed from materials such as copper, nickel, silver, gold, othermetals, a combination of these materials, or other suitable conductivematerial that suppress signals at radio frequencies and may sometimes bereferred to as radio-frequency shields or e-shields.

Shielding layer 304 directs magnetic fields at relatively lowerfrequencies to function as a guide for electromagnetic flux receivedfrom a wireless power transmitter. Layer 304 may be a layer of magneticmaterial that can serve as a magnetic shield (i.e., layer 304 can blockmagnetic flux and may have a relative permeability of 500 or more 1000or more, or other suitable value). An example of a material that can beused in forming magnetic shielding layer 304 is ferrite. Another exampleof a material that can be used in forming magnetic shielding layer 304is a high permeability nickel-iron magnetic alloy that is sometimesreferred to as mu-metal or permalloy. Another example of a material thatcan be used in forming magnetic shielding layer 304 is an iron-basednano-crystalline material.

In accordance with some embodiments, power transmitting device 12 caninclude one or more magnets that may contribute to certaincharacteristic conditions in a shielding structure within powerreceiving device 24. As shown in FIG. 1B, power receiving device 24 mayinclude a shielding layer 304. During wireless power transmission,inverter 60 may drive AC current signals through coil 42. The AC currentflowing through coil 42 induces AC magnetic flux that can add to the DCmagnetic flux associated with the magnet within device 12. Thecombination of the AC and DC magnetic flux at the transmitting device 12can result in a characteristic condition such as saturation at shield304. Saturation occurs when an increase in applied magnetic field cannotfurther increase the magnetization of the material. Saturation can alsooccur at ferrite or nano-crystalline materials with high magneticsaturation or high AC flux. Saturation (e.g., magnetic saturation ormagnetic flux saturation) can cause a reduction in the amount of matedinductance between devices 12 and 24, impacting wireless chargingperformance. FIG. 3 illustrates a reduction in mated inductance causedby saturation. FIG. 3 plots the mated inductance value LTX as a functionof the current ITX flowing through wireless power transmitting coil 42.As shown by curve 110, a reduction in the mated inductance valueresulting from saturation translates to an increase in current ITX.

FIG. 4 is a timing diagram illustrating the behavior of transmit coilcurrent I_(TX) with and without saturation. Waveform 120 represents thebehavior of current ITX in the absence of saturation, whereas waveform122 represents the behavior of current ITX in the presence ofsaturation. As shown in FIG. 4, waveform 120 toggles at an inverterswitching frequency Fs with a period Ts that is equal to the inverse ofFs (e.g., duration Ts is equal to the reciprocal of the power carrierfrequency). Waveform 120 has relatively stable peaks and valleys fromcycle-to-cycle, which yields an expected energy level at the fundamentalswitching frequency Fs.

In contrast, waveform 122 exhibits much high peak current levels everyother cycle (as shown by elevated peaks 124) as a result of thesaturation and reduced mated inductance. Waveform 122 recovers torelatively lower peak current levels every other cycle (as shown bylowered peaks 126). Waveform 122 therefore exhibits significantly higherenergy levels at half the switching frequency Fs/2 with a period 2*Ts.This phenomenon where higher energy levels are present at some fractionof the switching frequency Fs, particularly sub-harmonics of Fs, isindicative of saturation.

Referring back to FIG. 2, measurement circuitry 41 may include a foreignobject detection (FOD) circuit such as FOD circuit 100 and/or asaturation detection circuit such as saturation detection circuit 102.Saturation detection circuit 102 may include an energy measurementcircuit configured to measure a value representing energy levels in theresonant tank at various frequency bands to determine whether saturationand therefore oscillation have occurred. Saturation detection circuit102 may also be configured to measure the DC voltage across capacitor70. A non-zero DC voltage across capacitor 70 does not necessarily implysaturation, but saturation will result in a non-zero DC bias voltageacross capacitor 70.

Ferrite or other magnetic saturation that can occur within powerreceiving device 24 and the resulting oscillations can potentially causecommunications to fail between devices 12 and 24. As described above,oscillations occur when the transmitted electromagnetic flux becomessufficiently high to induce saturation in power receiving device 24. Ina typical wireless charging system, upon startup, the transmit powerwill start ramping up from a low power level to a target power level. Asthe transmit power level is ramped up, saturation (and characteristicoscillations) may occur. Saturation may also occur or re-appear afterthe power ramp up phase, for example if the wireless power receiver ismoved relative to the wireless power transmitter during power transfer.This can also occur when certain environmental or operating conditionsuch as temperature changes.

FIG. 5 is a flow chart of illustrative steps for performing power rampup operations. At step 130, the inverter power supply voltage Vin may beset to an initial voltage level. As an example, voltage Vin may beinitialized to 9 V. This is merely illustrative. The inverter supplyvoltage Vin may be initialized to 4 V, 5 V, 6 V, 7 V, 8 V, 10 V, 11 V,1-10 V, or other starting voltage level.

At step 132, the phase of the AC drive signal output by inverter 60 maybe set to an initial phase amount. As an example, the phase of theinverter AC drive signal may be set to 90 degrees. A 90° phase maytranslate to a 25% duty cycle. This is merely illustrative. The AC drivesignal phase may be initialized to 45 degrees (e.g., a 12.5% dutycycle), to 60 degrees (e.g., a 16.7% duty cycle), to 120 degrees (e.g.,33.3% duty cycle), to 135 degrees (e.g., 37.5% duty cycle), to 80-100degrees, 70-110 degrees, 60-120 degrees, or other starting phase amount.

At step 134, the control circuitry such as controller 16M may determinewhether the max phase has been reached. The control circuitry maycompare the current phase level to the maximum phase level. As anexample, the maximum phase level may be set to 180 degrees, whichtranslates to a 50% duty cycle. This is merely illustrative. The maximumphase may be set to 160 degrees, 170 degrees, 190 degrees, 200 degrees,less than 180 degrees, more than 180 degrees, 120-180 degrees, 180-360degrees, 170-190 degrees, 160-200 degrees, 150-210 degrees, 140-220degrees, or other maximum phase amount.

If the maximum phase has not been reached (i.e., if the current phase isequal to the maximum phase limit), the control circuitry will increasethe phase of the AC drive signal by a phase step amount at block 136.The phase step amount may be 5 degrees, 10 degrees, 15 degrees, 20degrees, or other phase delta. The inverter AC drive signal phase can beincreased by increasing the duty cycle of the AC drive signal. If themaximum has been reached (i.e., if the current phase is equal to orgreater than the maximum phase limit), the control circuitry willincrease the inverter supply voltage Vin by a voltage step amount atblock 138. The voltage step amount may be 1 V, 0.5 V, 2 V, 1.5 V, 0.1 V,0.2 V 0.3 V, 0.1-2 V, or other voltage delta.

At step 140, the control circuitry will determine whether the transmitpower level has reached the target power level. The target power levelmay be 12 V, 13 V, 14 V, 15 V, 16 V, 17 V, 18 V, 9-18 V, equal to orgreater than 12 V, equal to or greater than 18 V, or other target powerlevel. If the target power level has not been reached, processing mayloop back to step 134 as indicated by path 141. If the target powerlevel has been reached, the power ramping is complete (step 142).

As described above, saturation can occur during the power ramp up phaseor after the power ramp up phase. In accordance with some embodiments,control circuitry 16 within power transmitting device 12 (see, e.g.,FIG. 1) can be used to perform saturation detection and mitigationduring the power ramp up phase and/or after the power ramp up phase. Ifno oscillation is detected during the power ramp up phase, then device12 can continue to ramp up its power level. FIG. 6 is a flow chart ofillustrative steps for performing saturation detection and mitigationoperations.

At step 200, data receiver 40R may receive a control error packet (CEP),the communications between devices 12 and 24 may time out, or asaturation detection timer may expire. The Qi mechanism for controllingthe transmit power level uses power receiving device 24 to send to powertransmitting device 12 power adjustment requests such as ASK modulatedpackets sometimes referred to as a control error packet (CEP). Controlcircuitry 16 may include an saturation detection timer that expires totrigger a corresponding saturation detection operation. The saturationdetection timer can be started periodically or in response to certainevents such as the start of the power ramp up phase.

In response to power transmitting device 12 receiving a control errorpacket from power receiving device 24, in response to a communicationstime out event, or in response to the saturation detection timerexpiring, saturation detection circuit 102 (see, e.g., FIG. 2) may beconfigured to perform saturation detection operations at step 202.Various saturation detection schemes can be used.

As an example, saturation detection circuit 102 can include ameasurement circuit configured to measure an energy level of theresonant tank or a value representing the energy level such as ameasured current level or a measured voltage level at a measurementfrequency that is equal to half the inverter switching frequency (e.g.,the measurement frequency may be equal to Fs/2). Measurement circuit 102is therefore sometimes referred to as an energy measurement circuit. Theenergy measurement circuit may be a frequency selective energycomputation block having a bandpass filter followed by an energyintegrator (as an example). As another example, the energy measurementcircuit may include a fast Fourier transform (FFT) block. Saturationdetection circuit 102 may compare the measured value to a threshold.

The threshold may be equal to one percent of an energy level or anothervalue representing the energy level of the resonant tank at switchingfrequency Fs. The energy level at frequency Fs can be an anticipatedamount of energy generated by the AC drive signal at the output ofinverter 60 in the absence of saturation (e.g., the expected energylevel at Fs generated by waveform 120 in FIG. 4). The anticipated(expected) amount of energy can be predetermined using simulation orexperimentally. The energy level at switching frequency Fs can also bemeasured in real time using measurement circuit 102 (e.g., by tuning thebandpass filter to Fs). This 1% threshold is merely illustrative. Inother embodiments, the threshold may be equal to 0.1% of theexpected/measured energy level at Fs, 0.1-1.0% of the expected/measuredenergy level at Fs, 2% of the expected/measured energy level at Fs, 1-5%of the expected/measured energy level at Fs, 1-10% of theexpected/measured energy level at Fs, less than 1% of theexpected/measured energy level at Fs, more than 1% of theexpected/measured energy level at Fs, or other desired fraction of theenergy level at Fs. If the measured value exceeds the threshold, thensaturation has been detected. If the measured value does not exceed thethreshold, then saturation has not been detected and saturationdetection terminates (at step 204).

The example above in which the measurement circuit measures the energylevel (or some value representing the energy level) at Fs/2 is merelyillustrative. As another example, the measurement circuit might measurean energy-representative value at Fs/3. As another example, themeasurement circuit might measure an energy-representative value at2*Fs/3. As another example, the measurement circuit might measure anenergy-representative value at Fs/4. As another example, the measurementcircuit might measure an energy-representative value at 3*Fs/4. Ingeneral, saturation detection circuit 102 can be configured to measurean energy-representative value (e.g., a measured current value or ameasured voltage value) at any suitable sub-harmonic range or fractionof switching frequency Fs.

The example above in which the energy measurement circuit measures avalue representing the energy level at some fraction of switchingfrequency Fs is merely illustrative. As shown in FIG. 3, the lower peaks126 of waveform 122 showing saturation can excite energy at the evenharmonics. Thus, the measurement circuit might measure the energy levelat 2*Fs, 4*Fs, 6*Fs, and so on and compare the measured energy level tosome threshold that is some fraction of the expected energy level at Fs.If desired, the saturation detection circuit 102 can be configured tomeasure the energy level at odd harmonics of the switching frequency(e.g., 3*Fs, 5*Fs, 7*Fs, and so on).

The examples above in which the saturation detection circuit 102measures energy levels in various frequency sub-bands is merelyillustrative. In other embodiments, circuit 102 can perform saturationdetection in the time domain. For example, saturation detection circuit102 may measure the peak to peak variation over N 2 cycles and comparethe peak measured during one cycle to the peak measured during asubsequent cycle (e.g., by computing a ratio of the peak values measuredfrom at least two consecutive cycles). Saturation detection circuit 102may monitor peak-to-peak current, peak-to-peak voltage, and/orpeak-to-peak power levels.

As shown in FIG. 4, the peak-to-valley variation in waveform 122 can befairly large from one cycle to another when saturation is present. Forinstance, a first delta value can be obtained by computing thedifference between the peak and valley during a first cycle, whereas asecond delta value can be obtained by computing the difference betweenthe peak and valley during a second cycle following the first cycle. Ifthe maximum delta value or if the variance of the two delta values overN cycles exceeds a delta threshold level, then saturation is detected.If the maximum delta value or if the variance of the two delta valuesover N consecutive cycles does not exceed the delta threshold level,then saturation has not been detected and saturation detectionterminates (at step 204). This time domain peak-to-peak variation canalso be computed by applying a smoothing filter (e.g., using a slidingaverage window). The threshold level used during time domain saturationdetection may be a deterministic threshold value that is identifiedexperimentally or via simulation.

The examples above in which the saturation detection circuit 102measures energy levels at one or more frequencies is merelyillustrative. As another example, measurement circuitry 41 may use aseparate indicator of lost energy as a proxy for saturation. Saturationcan lead to excessive energy losses, which can inadvertently trigger FODand can lead to shut down. To prevent FOD from being inadvertentlytriggered, a blanking timer may be used to temporality deactivate FODcircuit 100 during the power ramp up phase or during saturationdetection operations. As yet another example, measurement circuitry 41may be configured to measure the DC bias voltage across the seriescapacitor (see capacitor 70 in FIG. 2). When saturation occurs, anon-zero bias voltage is seen across the series capacitor.

In the example of FIG. 6, M contiguous positive saturation detectionsmay be required at step 202 before proceeding with the saturationmitigation operations. M may be equal to one, two, three, four, five,1-5, more than one, more than five, 5-10, or other integer. Higher Mvalues can help filter out potentially noisy saturation measurements andprevent false positive saturation detection.

If saturation is detected, various saturation mitigation operations canbe performed (see, e.g., steps 206, 208, 210, and/or 212 in FIG. 6). Atstep 206, the control circuitry may reduce the phase (e.g., the dutycycle) of the AC drive signal until a minimum phase is reached or untilsaturation is no longer detected. The minimum phase may be equal to 70degrees, less than 70 degrees, more than 70 degrees, 60-80 degrees,50-90 degrees, or other phase amount. For example, the control circuitrymay decrease the phase by 5° and re-perform saturation detection tocheck whether saturation has been mitigated. The 5° step size is merelyillustrative. If desired, a phase step size of less than 5°, more than5°, 1-5°, 5-10°, 1-10°, or other phase delta can be used. If desired,phase may decrease more rapidly at higher phase levels and decrease moregradually at lower phase levels. Once saturation is no longer detected,saturation mitigation operations are complete.

If the minimum phase has been reached but saturation is still present,the control circuitry may reduce the inverter supply voltage Vin untilsaturation is no longer detected (step 208). For example, the controlcircuitry may decrease voltage Vin by 200 mV and re-perform saturationdetection to check whether saturation has subsided. The 200 mV step sizeis merely illustrative. If desired, a voltage step size of 10 mV, 50 mV,100 mV, 300 mV, 10-300 mV, 190-210 mV, 180-220 mV, 150-250 mV, 100-300mV, or other voltage delta can be used. Once saturation is no longerdetected, saturation mitigation operations can be terminated.

As another example, the control circuitry can optionally adjust theswitching frequency of the AC drive signal until saturation is no longerdetected (step 210). Adjusting the switching frequency (e.g., increasingor decreasing Fs) can reduce the coupling gain as well as the half-cycleperiod, which can help limit the increase in the transmit coil currentand thus prevent saturation. It is also possible to change powertransmission levels to detect if power transmission wattage levelsaffect saturation. For example, the Qi standard allows for differentpower profiles. In some implementations a wireless power transmitter mayaccount for saturation in determining whether to operate under, forexample, base or extended power profiles.

As another example, the control circuitry can optionally operateinverter 60 using an asymmetric switching scheme to mitigate saturation(step 212). FIG. 7 shows inverter 60 driving a resonant tank circuit 72having coil 42 connected in series with capacitor 70. As shown in FIG.7, inverter 60 (e.g., a full-bridge inverter) may include switches S1and S2 coupled in series between the Vin supply and ground and mayinclude switches S3 and S4 coupled in series between the Vin supply andground. Resonant tank has one terminal that is connected to a firstswitch node N1 interposed between inverter switches S1 and S2 and hasanother terminal that is connected to a second switch node N2 interposedbetween inverter switches S3 and S4.

FIG. 8A is a timing diagram illustrating an inverter output behaviorwith a 180° phase shift and a symmetric switching duty cycle. As shownin FIG. 8A, node N1 is driven high to supply voltage Vin for a durationT1 that is equal to half the inverter switching period Ts/2. The timedelay between the rising edge of N1 and the rising edge of N2 is definedas the phase shift and is equal to 180° in this example. After the 180°phase delay, node N2 is driven high to supply voltage Vin for a durationT2 that is equal to Ts/2. The third waveform shows the result of N1minus N2, which is the driving voltage applied to resonant tank 72. Theresult is a positive Vin for duration T1 followed by a negative Vin forduration T2. FIG. 8A shows how both N1 and N2 have equal durations, thusresulting in a symmetrical switching waveform (symmetrical duty cycle)on N1 minus N2 where the duration of +Vin is equal to the duration of−Vin.

A symmetrical excitation by the inverter typically results in asymmetric resonance waveform. In particular, the voltage waveform acrossseries capacitor 70 will be symmetrical in the two half switchingperiods and average out to zero. However, if the ferrite structure orother magnetically permeable material in device 24 is saturated bynearby DC magnets, the permeability of such material will decrease asthe resonant current moves in one direction and increase the resonantcurrent moves in the other direction. This causes a resonant inductancevalue that is different in the two half switching periods. Ifhypothetically the voltage across capacitor 70 initially remainssymmetrical in the two half switching periods, the voltage across coil42 would remain symmetrical in the two half switching periods, thus thevarying inductance of coil 42 would cause unequal currents in the twohalf switching periods. The unequal currents would move capacitor 70′saverage voltage away from zero. A new equilibrium state will beestablished when the voltage across capacitor 70 (sometimes referred toherein as Vctx) reaches an average voltage level (sometimes referredherein as DC bias) that restores the charge balance condition forcapacitor 70. As a result, magnetic saturation (e.g., ferritesaturation) causes a DC bias in Vctx even though the inverter excitationis symmetrical.

In accordance with an embodiment, removing such DC bias in Vctx can helpremove oscillation caused by saturation. A non-zero DC bias in Vctx, asdetected using measurement circuitry 41 (see FIG. 2), can trigger afeedback control mechanism that adjusts inverter 60 in a way that drivesthe average Vctx towards zero. As an example, in response to usingmeasurement circuitry 41 to detect a non-zero DC bias in Vctx, controlcircuitry 16M (see FIG. 2) can adjust inverter 60 to reduce the dutycycle of the AC drive signals output by inverter 60. Reducing the dutycycle of the inverter output signals can help drive average Vctx towardszero to help mitigate saturation. Other ways of adjusting inverter 60 toreduce average Vctx can also be used.

As another example, the unwanted DC bias in Vctx can be removed byapplying opposite offsets to the duty cycles of nodes N1 and N2. FIG. 8Bis a timing diagram showing an inverter output with a 180° phase shiftand an asymmetric switching duty cycle. Compared to FIG. 8A, node N1 isdriven high to supply Vin for a modified duration T1′ that is lengthenedby offset Toffset while node N2 is driven high (after a phase shiftPhase') to supply Vin for a modified duration T2′ that is shortened byoffset Toffset. The phase shift time of node N2 (as denoted by Phase')effectively becomes (Ts/2+Toffset). Here, Toffset is shown as a positivevalue, but Toffset can also be a negative value. This results in adifferent waveform (e.g., N1 minus N2) having a +Vin for duration T1′and −Vin for duration T2′. This behavior in which the duty cycle ofnodes N1 and N2 are different is sometimes referred to herein as aninverter switching operation with an asymmetric duty cycle.

FIG. 8C is a timing diagram showing an inverter output with a 90° phaseshift and an asymmetric switching duty cycle. Compared to FIG. 8B, nodeN1 is driven high to supply Vin for a modified duration T1′ that islengthened by offset Toffset while node N2 is driven high (after a phaseshift Phase″) to supply Vin for a modified duration T2′ that is againshortened by offset Toffset. The phase shift time of node N2 (as denotedby Phase″) effectively becomes (Ts/4+Toffset). Here, Toffset is shown asa positive value, but Toffset can also be a negative value. This resultsin a different waveform (e.g., N1 minus N2) having a +Vin for durationT1″ and −Vin for duration T2″. This behavior in which the duty cycle ofnodes N1 and N2 are different is sometimes referred to herein as aninverter switching operation with asymmetric duty cycle.

The offset Toffset can be computed by a compensator block within controlcircuitry 16 (FIG. 1), taking the Vctx DC bias value as a negativefeedback input. When the DC bias is negative (as defined by the Vctxpolarity shown in FIG. 7), the offset will be a positive value thatlengthens T1 while shortening T2. When the DC bias is positive, theoffset will be a negative value that shortens T1 while lengthening T2.This compensator block can take various forms, such as aproportional-integral-derivative (PID) controller, aproportional-integral (PI) controller, or a simple integrator. Thecompensator block should include an integral component as the saturationmitigation loop needs to retain a Toffset value even when the DC bias isdriven to zero.

As shown in the example of FIG. 6, the phase is decreased until someminimum phase amount and then voltage Vin is reduced. As anotherexample, the control circuitry may lock or fix the voltage level of Vinas soon as saturation is detected. This prevents the supply voltage Vinfrom further increasing, thereby attenuating one of the causes ofsaturation. The examples herein for mitigating saturation in response todetecting saturation caused by saturation is merely illustrative. Ingeneral, the various embodiments for mitigating saturation can also beapplied in response to detecting when a magnetically permeable materialexceeds its magnetic saturation level.

The foregoing is merely illustrative and various modifications can bemade to the described embodiments. The foregoing embodiments may beimplemented individually or in any combination.

What is claimed is:
 1. A wireless power transmitting device, comprising:a resonant circuit having a wireless power transmitting coil configuredto transmit wireless power to a wireless power receiving device; aninverter configured to drive alternating current signals at a switchingfrequency onto the wireless power transmitting coil; a measurementcircuit configured to measure a value representing an energy level inthe resonant circuit; and control circuitry configured to: detectsaturation in the resonant circuit by comparing the measured value to athreshold; and control wireless power transmission via the wirelesspower transmitting coil in response to detecting the saturation.
 2. Thewireless power transmitting device of claim 1, wherein the measuredvalue comprises a measured current level.
 3. The wireless powertransmitting device of claim 1, wherein the measured value comprises ameasured voltage level.
 4. The wireless power transmitting device ofclaim 1, wherein the measurement circuit is configured to measure thevalue at a measurement frequency that is in a sub-harmonic range of theswitching frequency.
 5. The wireless power transmitting device of claim4, wherein the measurement frequency is half of the switching frequency.6. The wireless power transmitting device of claim 1, wherein themeasurement circuit is configured to measure the value at a measurementfrequency that is a harmonic of the switching frequency.
 7. The wirelesspower transmitting device of claim 6, wherein the measurement frequencyis an even multiple of the switching frequency.
 8. The wireless powertransmitting device of claim 1, wherein: the measurement circuit isconfigured to measure an additional value representing an energy levelin the resonant circuit at the switching frequency; and the threshold isa fraction of the additional value.
 9. The wireless power transmittingdevice of claim 1, wherein: the measured value comprises a measuredcurrent level or a measured voltage level; and the control circuitrydetects the saturation by comparing a variance of the measured value tothe threshold.
 10. The wireless power transmitting device of claim 1,wherein: the measured value comprises a peak-to-valley difference; andthe control circuitry detects the saturation by comparing a variance ofthe peak-to-valley difference to the threshold.
 11. The wireless powertransmitting device of claim 1, wherein: the measured value comprises apeak-to-valley difference; and the control circuitry detects thesaturation by comparing the peak-to-valley difference to the threshold.12. The wireless power transmitting device of claim 1, wherein: themeasured value comprises a measured current level or a measured voltagelevel; and the control circuitry detects the saturation by comparing aratio of the measured value in two consecutive cycles to the threshold.13. The wireless power transmitting device of claim 1, wherein thecontrol circuitry is configured to control the wireless powertransmission via the wireless power transmitting coil in response todetecting the saturation by reducing a duty cycle of the alternatingcurrent signals driven onto the wireless power transmitting coil. 14.The wireless power transmitting device of claim 1, wherein the controlcircuitry is configured to control the wireless power transmission viathe wireless power transmitting coil in response to detecting thesaturation by reducing a power supply voltage powering the inverter. 15.The wireless power transmitting device of claim 1, wherein the controlcircuitry is configured to control the wireless power transmission viathe wireless power transmitting coil in response to detecting thesaturation by limiting the wireless power to a predetermined power levelto mitigate the saturation.
 16. The wireless power transmitting deviceof claim 1, wherein the control circuitry is configured to control thewireless power transmission via the wireless power transmitting coil inresponse to detecting the saturation by adjusting the switchingfrequency of the inverter.
 17. The wireless power transmitting device ofclaim 1, wherein: the resonant circuit further comprises a capacitorcoupled in series with the wireless power transmitting coil; and thecontrol circuitry is configured to control the wireless powertransmission via the wireless power transmitting coil in response todetecting the saturation by operating the inverter using asymmetric dutycycle to remove a DC bias voltage across the capacitor.
 18. The wirelesspower transmitting device of claim 1, wherein the control circuitry isconfigured to: in response to not detecting the saturation, increase thewireless power.
 19. The wireless power transmitting device of claim 18,further comprising: a foreign object detection circuit configured todetect whether a foreign object is present on the wireless powertransmitting device, wherein the foreign object detection circuit isdeactivated while the control circuitry is increasing the wirelesspower.
 20. The wireless power transmitting device of claim 1, whereinthe control circuitry is configured to: after detecting the saturation,receive a request from the wireless power receiving device to increasethe wireless power; and ignore the request until saturation is no longerdetected.
 21. A wireless power transmitting device, comprising: awireless power transmitting coil configured to transmit wireless powerto a wireless power receiving device; an inverter configured to drive analternating current signal onto the wireless power transmitting coil;and control circuitry configured to: detect a characteristic conditionat a magnetically permeable structure of the wireless power receivingdevice; and in response to detecting the characteristic condition at themagnetically permeable structure of the wireless power receiving device,adjust the inverter.
 22. The wireless power transmitting device of claim21, wherein: the magnetic permeable structure comprises a ferritestructure; and the characteristic condition comprises ferrite saturationat the ferrite structure.
 23. The wireless power transmitting device ofclaim 21, wherein: the control circuitry is configured to detect thecharacteristic condition by measuring a current or voltage level andcomparing the measured current or voltage level to a threshold.
 24. Thewireless power transmitting device of claim 21, wherein the controlcircuitry is configured to adjust the inverter by reducing a duty cycleof the alternating current signal driven onto the wireless powertransmitting coil in response to detecting the characteristic condition.25. The wireless power transmitting device of claim 21, wherein: theinverter is configured to receive a power supply voltage; and thecontrol circuitry is configured to adjust the inverter by reducing thepower supply voltage in response to detecting the characteristiccondition.
 26. The wireless power transmitting device of claim 21,wherein: the inverter is configured to receive a power supply voltage;and the control circuitry is configured to fix the power supply voltagein response to detecting the characteristic condition.
 27. The wirelesspower transmitting device of claim 21, wherein: the inverter isconfigured to drive the alternating current signal at a switchingfrequency; and the control circuitry is configured to adjust theinverter by adjusting the switching frequency.
 28. The wireless powertransmitting device of claim 21, further comprising: a foreign objectdetection circuit configured to detect whether a foreign object ispresent on the wireless power transmitting device, wherein: the inverteris configured to receive a power supply voltage; and the foreign objectdetection circuit is deactivated while the control circuitry is rampingup the power supply voltage.
 29. A wireless charging system, comprising:a wireless power receiver having a magnetically permeable material; anda wireless power transmitter configured to transmit wireless power tothe wireless power receiver, the wireless power transmitter having: atank circuit with a wireless power transmitting coil configured totransmit the wireless power to the wireless power receiving device; aninverter configured to drive alternating current signals at a switchingfrequency onto the wireless power transmitting coil; and controlcircuitry configured to: detect a characteristic condition at themagnetically permeable material in the wireless power receiving device;and in response to detecting the characteristic condition, controlwireless power transmission via the wireless power transmitting coil.30. The wireless charging system of claim 29, wherein the characteristiccondition comprises saturation at the magnetically permeable material.31. The wireless charging system of claim 29, wherein: the magneticallypermeable material comprises ferrite; and the characteristic conditioncomprises ferrite saturation at the ferrite.
 32. The wireless chargingsystem of claim 29, wherein the characteristic condition causesoscillation in the tank circuit.
 33. The wireless charging system ofclaim 29, wherein: the wireless power transmitter comprises ameasurement circuit configured to measure a value representing an energylevel in the tank circuit at a measurement frequency that is a functionof the switching circuitry; and the control circuitry is configured todetect the characteristic condition by comparing the measured value to athreshold.
 34. The wireless charging system of claim 29, wherein: thewireless power transmitter comprises a measurement circuit configured tomeasure a peak value; and the control circuitry is configured to detectthe characteristic condition by comparing a metric that is function ofthe measured peak value to a threshold.
 35. The wireless charging systemof claim 29, wherein: the wireless power transmitter comprises a foreignobject detection circuit configured to detect whether a foreign objectis present on the wireless power transmitting device; and the foreignobject detection circuit is deactivated while the control circuitry isdetecting the characteristic condition.